[vdr] Re: Automatic channel scan

["Oleg Assovski" <assen@bitcom.msk.ru>]

Sven Karlsson writes: 

> I'm just writing this to say that I've almost succeeded in letting VDR
> automatically scan DVB channels. At least on DVB-C. 
> 
> The STBes for DVB-C usually only need one frequency and the corresponding
> symbolrate and modulation parameters to sucessfully find all other
> "transponders"(I not sure about the nomenclature). However, neither VDR nor
> DTV can do this. I've made some changes, however, to VDR as to parse the
> network identification table(NIT) as sent in the digital feed. The NIT
> contains descriptors for frequences and other goodies like network name and
> more. 
> 
> I can now successfully find all "transponders" given just one initial
> frequency (just like the STBes) and I can also obtain all "non-digital"
> channel frequences as these also appears to be passed in the NIT. It is of
> course straight forward to find all channels given the transponders.

There is a great difference between DVB-C and DVB-S. Simply said DVB-C is 
transmitted as one bouquet (package) of channels per cable. And you can grab 
all the NIT data from one "base" frequency.
However DVB-S universe is composed of several bouquets, multiplexes (single 
transponder with multiple channels), and even single channel transponders 
(SCPC). So you have as many "base" frequencies as you have different 
bouquets, muxes and SCPC transponders. That is why dtv_scan uses external 
frequency list for channel scan. It's simpler. 

> 
> The STBes I've seen can also allocate the different channels to "channel
> numbers" so that a specific channel can always be seen on "channel x" on the
> STB although the channel moves between transponders. The providers tend to

There are two straightforward ways to do it described in standard. Provider 
can use either OrigianlNetworkID/ServiceID pair or channel name. There are 
other possibilities like using Linkage descriptors, custom descriptors etc 
etc but these will not be compaitible between providers. Moreover 
CanalSatellite France transmits custom table that maps channel numbers to 
ONID/TSID/SIDs but these channel numbers do not correspond to channel 
numbers seen on their site. :( 

> I would very much like to know if someone else is working on something
> similar or if all this has already been done.

 From my personal experience I can ensure you that in theory it is possible 
to write DVB compliant satellite scanner. You will feed it with huge 
frequency list and you will get this list with refined and enriched data.
As I've wrote already you can see this working at 
http://www.satmania.com/eng
But unfortunately most providers (even almost all) provides inconsistent 
data in NITs. These includes bad modulations, polarisations, frequencies, 
NID/TSIDs, orbital positions. Almost each position in NIT can be faked. And 
the biggest part of my work when implementing the scanner was to ensure 
protection of database from contamination by bad data. 

         Oleg Assovski
 ----------------------------------------
We make few little bugs
We compensate with a lot of big ones